This invention relates to a method for reducing chemical background in electrospray and nanospray mass spectra. More specifically, this invention relates to a computer-based method for reducing the component attributed to chemical background in acquired mass spectra.
The mass spectrometer is an instrument that is used to establish the molecular weight and structure of organic compounds, and to identify and determine the components of inorganic substances. Presently, there are known a large number of different mass spectrometers, such as quadrupole, magnetic sector, Fourier transform ion cyclotron resonance (FTICR), and other multipole spectrometers and Time-of-Flight (TOF) devices. All of these, fundamentally, require sample molecules to be ionised. There are a variety of conventional techniques for converting an initially neutral sample into an ionized species in the gas phase. These ions are then separated in the mass spectrometer according to their mass/charge (m/z) ratios. For example, electrospray and nanospray techniques are particularly useful in mass spectrometry of macro molecular compounds. These ions are then typically detected electrically by the mass spectrometer, at which time the ion-currents corresponding to the different elements or compounds which comprise the sample can be measured. This information can then be stored, for example, in a computer for subsequent processing and analysis.
In mass spectrometry, it is well-known that many organic and inorganic samples may contain some quantity of undesirable compounds which are not the subject of study, but which were not removed in the process of preparing the samples for analysis. The undesirable compounds may also be contaminants that have found their way into the mass spectrometer during the sample introduction phase. These undesirable compounds subsequently produce chemical background in acquired mass spectra. For atmospheric pressure sources, the potential contaminants include gases.
The precise nature of chemical background is difficult to determine. Chemical background may be formed by all possible combinations of (CnAm)+k, where C and A are cations and anions respectively, of different contaminant elements and compounds originating from the sample itself or from the sample introduction system, presented in combination n, m, and having charge k.
Various methods have been proposed in the art for removing these contaminants. The prior art system disclosed in U.S. Pat. No. 5,703,358 issued to Hoekman et al. contemplates a method for generating a filtered signal which can be applied in mass spectrometry experiments. The system disclosed in Hoekman et al. enables the rapid generation of filtered noise signals, (e.g., in real time during mass spectrometry experiments) without prior knowledge of the mass spectrum of unwanted ions to be ejected from an ion trap during application of the filtered noise signal to the ion trap. The system disclosed in Hoekman et al. does not appear to deal with the elimination of chemical background using spectrometry data already acquired.
The prior art method and apparatus disclosed in U.S. Pat. No. 5,324,939 issued to Louris et al. provides a method and apparatus for selectively ejecting a range of ions in an ion trap while retaining others. This method and apparatus does not appear to deal with the elimination of chemical background using spectrometry data already acquired.
The prior art method and apparatus disclosed in U.S. Pat. No. 4,761,545 issued to Marshall et al., provides a method and apparatus for excluding a range or ranges of ions from detection within an ion cyclotron resonance cell. This method and apparatus involves the ejection of unwanted ions from the cell, and does not appear to deal with the elimination of chemical background using spectrometry data already acquired.
These prior art systems and methods may succeed in eliminating contaminants with different mass/charge ratios, but they typically cannot remove contaminants having a mass/charge ratio similar to that of an ion of interest. Therefore, they cannot be used to filter out non-spectral interferences.
However, there is still a need to reduce or eliminate chemical background in post-experiment acquired mass spectra, so as to provide for a better signal-to-noise ratio, greater mass accuracy, and to improve the overall presentation of information relating to the sample, allowing for easier comprehension and analysis. More particularly, there is a need to filter out non-spectral interferences covering a wide range of mass/charge ratios.
There is also a need for a rapid, efficient, and automated process for reducing or eliminating chemical background from a given mass spectrum. Further, there is a need for a method which can process data already obtained from a mass spectrometer without having to perform additional experiments using the mass spectrometer or to make subsequent adjustments to the mass spectrometer, to obtain a mass spectrum with reduced chemical background.
There is also a need for reducing or eliminating chemical background in real-time, as data is being acquired from a mass spectrometer or shortly thereafter.
The invention provides for a method of reducing chemical background from a mass spectrum comprising the steps of obtaining a mass spectrum including both data for desired ions of interest and a chemical background, determining the presence of chemical background in the mass spectrum and determining at least one dominant frequency of the chemical background, and filtering out at least one dominant frequency whereby at least a substantial portion of the chemical background is removed from the mass spectrum.